Cluster ion formation in laser mass spectrometry of substituted pyridines

Emission of cluster ions occurs during laser irradiation of substituted pyridines even at threshold laser power densities. The clusters generated include dimers and trimers, and appear in both positive-ion and negative-ion laser mass spectra. Fragments of cluster ions are observed and can be rationlized as losses of neutral molecules from (nM ± H)^±. Dissociation of clusters occurs primarily from substituents on the pyridine ring. Laser mass spectrometry of pyridoxine hydrochloride and pyridoxamine-dihydrochloride resulted in the emission of clusters analogous to those observed for nicotinic acid. In contrast to these results, secondary-ion and field-desorption mass spectra of salts contain the ions C_nA_n?1⁺ and C_nA_n+1^?, that were not detected in the laser mass spectra.